Scientists Discover Light, Gold Thin Films Interaction

Physicists have devised a way to determine the electronic properties of thin gold films after they interact with light, a discovery that could further scientific understanding of how light affects materials.

When the energy of a photon is transferred to an electron in a light-absorbing material, the photon is destroyed and the electron is excited from one level to another. Because many photons, atoms, and electrons are involved, and the process happens very quickly, laboratory modeling of this process is computationally challenging.

Developing a fundamental understanding of ultrafast nonthermal processes in metallic nanosystems could lead to applications in photodetection, photochemistry, and photonic circuitry. Courtesy of Emory University.Using experiments and a theoretical model to decode the experimental data, physicists from Emory University directly determined nonthermal and thermal distributions and dynamics in thin films. They applied a double inversion procedure to optical pump-probe data, relating the reflectivity changes around Fermi energy to the changes in the dielectric function and in the single-electron energy band occupancies.

'Optical phenomenon is such a fundamental process that we take it for granted, and yet it’s not fully understood how light interacts with materials,' said physicist Hayk Harutyunyan.

When the Emory team’s method was applied to normal incidence measurements, it revealed the ultrafast excitation of a non-Fermi-Dirac distribution and subsequent thermalization dynamics. When the team applied its approach to the Kretschmann configuration, results showed that the excitation of propagating plasmons could lead to a broader energy distribution of electrons, due to the enhanced Landau damping.

In experiments, light was shined on gold nanolayers using two extremely short pulses. The first pulse was absorbed by the gold. The second pulse measured the results of the absorption, revealing how the electrons changed from a ground state to an excited state.

Gold typically absorbs light at green frequencies; but when it is in the form of nanolayers, it can absorb light at longer wavelengths.

“At a certain excitation angle, we were able to induce electronic transitions that were not just a different frequency but a different physical process," said professor Hayk Harutyunyan. "We were able to track the evolution of that process over time and demonstrate why and how those transitions happen.”

Scientists could use this method to help them better understand the interactions between light and materials that underlie the absorption of light into a material. This, in turn, could lead to ways to tune and manage the interactions between light and materials — for example, between light and photovoltaic energy cells or between light and optical sensors.

The researchers plan to continue to refine their method’s use with gold while also experimenting with other materials.

“Ultimately, we want to demonstrate that this is a broad method that could be applied to many useful materials,” said Harutyunyan.

The research team at Emory collaborated with researchers at Argonne National Laboratory and Ohio University.